The present invention relates to a method and a circuit arrangement for producing mixed light of a predetermined colour by mixing of light with a longer wavelength being emitted by at least one first LED with light with a shorter wavelength being emitted by at least one second LED. The boundary between the light with a longer wavelength and the one with a shorter wavelength can be e.g. at 500 nm (regarding to the spectrum peak).
It is known to produce mixed light of a predetermined colour by mixing light emitted by at least two LEDs, the light emitted by the one LED and the light emitted by the other LED having different wavelengths. White light can for example be produced by mixing the light emitted by a red light LED and the light emitted by a colour-converted blue light LED (this is e.g. an LED chip producing blue light or UV light which is covered by a phosphor film converting the blue light or the UV light into light with a longer wavelength having a corresponding different colour).
Alternatively white light can be produced by RGB (red, green, blue) mixing.
However, there occurs a problem in that, the colour locus of the mixed light in the CIE chart changes along with the temperature. A cause for the temperature change can be fluctuations of the ambient temperature, but also that the LED module warms up due to the operating current with the elapse of time. In the latter case a steady state is achieved only once a certain warm up time has passed. Generally this is at least 10 minutes, but can be considerably longer.
Temperature changes result in colour locus changes of the mixed light for the following reason: The higher the temperature within an LED module raises the lower is the intensity of the light emitted by the LED (with constant current through the LED). The intensity curve in dependency of the temperature is sloping—or in other words—the gradient is negative. As such, this would not be a problem in itself regarding to the colour of the mixed light, if the gradient of the LED light with the longer wavelength and that of the LED light with the shorter wavelength were the same. Actually, the negative gradient of light with longer wavelengths is greater than the negative gradient of LED light with shorter wavelengths resulting in a variation in the spectrum of the mixed light.
Thus, a typical warming up of an LED module e.g. from room temperature to 60° C. to 80° C. can result in a colour locus shift which is perceptible by the human eye.